This invention relates to a flapper valve comprising a valve chamber with an outlet orifice and a piezoelectric flapper element which is mounted by one end in structure of the valve chamber in the manner of a cantilever so that it extends into the valve chamber, the flapper element being operable to bend and interact with fluid flow through a nozzle which is formed in a wall of the valve chamber whereby to modulate fluid flow through that nozzle when an electrical potential is applied to it.
SE-A-322989, WO80/01826, GB-A-2137776, GB-A-2181278 and DE-A-3608550 each disclose the use of a tongue-shaped piezoelectric element to regulate air or gas pressure in a line by controlling airflow from the line through a nozzle. The piezoelectric element is supported at one end in the manner of a cantilever and has its other end co-operating with the nozzle. The element is caused or allowed to bend laterally by the application of an electrical potential to it so that part of it moves towards or away from the nozzle whereby flow of air through the nozzle is modulated, that is to say the flow through the nozzle is regulated by the proximity of the free end of the flapper element to the nozzle.
GB-A-2137776 describes the piezoelectric element as consisting of a piece of thin brass conductive strip sandwiched between two layers of ceramic material, one of which is caused to expand and the other to contract when an electrical potential is applied across the layers. Such elements are known as bimorph piezoelectric elements, or flappers.
Whereas SE-A-322989, WO80/018216 and GB-A-2137776 only describe the use of a flapper to modulate or regulate airflow through a nozzle and thereby to control pressure in a line leading to a nozzle, each of GB-A-2181278 and DE-A-3608550 describes the use of such a bimorph flapper to close or open a port with which it co-operates. The flapper disclosed by GB-A-2181278 closes the port by seating at the downstream end of the port. DE-A-3608550 discloses a three-way valve having two such flapper elements, one of which closes one of the ports at its downstream end so that the other two ports are connected, and the other of which closes one of the other two ports at its upstream end whilst the first mentioned flapper element is unseated so that the port with which it cooperates is connected to the third port. GB-A-2137776 describes the flapper as being trapezoidal in form, the shorter of the two parallel sides being at its free end.
The use of such a bimorph flapper to close the port by seating at the upstream end of the port is disclosed in an article entitled "High-Speed Pulsed Valve Based on a Bimorph Piezoelectric Element" by V. N. Garnov et al and published in "Instruments and Experimental Techniques" Vol. 23 (1980) July-August No. 4, Part 2, New York, U.S.A. The upstream end of the port is formed in a flat surface at the end of a tubular boss which projects into the valve chamber. The flapper carries a rubber sealing gasket which seats on the flat surface and which is compressed by deflection of the flapper when so seated to make the seal. The seal is a face seal, the gasket and the flat surface having substantially the same circumference and diameter. It has been found that this valve is not a truly, normally-closed valve. To the contrary, there is leakage due to a continual opening and shutting of the valve which is accompanied by a blowing-off noise like that made by air escaping from a toy balloon when the latter is released. Such repetitive opening and closing of the valve can lead to resonance of the flapper which can induce catastrophic high voltages which will reduce the lifetime of the flapper and/or the drive electronics. DE-A-3608550 discloses the provision of a flexible sealing disc at the free end of each of its flapper elements, the sealing disc being caused to fit tightly against a valve seat formed at the end of the respective port and having a larger area than that valve seat which comprises a flat surface which is formed around the mouth of the outlet and which in turn is surrounded by a conically bevelled semi-spherical or other profiled surface. The seal is shown as a face seal between the sealing disc and the flat valve seat.
Limitations of such previously known bimorph piezoelectric flapper valves have been the power requirements, the order of actuation force required and the distances through which the free end of the flapper element needs to be moved as well as high hysteresis and limited stiffness of the flapper elements.